A seat belt is wound on a belt reel, for example an automatic belt retractor. A belt tightener can be brought into driving connection with the belt reel in order to eliminate belt slack from the wound seat belt and/or the belt webbing extending across the vehicle occupant. For example in the event of a crash the belt tightener assures that the seat belt rests firmly on the vehicle occupant's body and excessive forward displacement is prevented. A driving part of the belt tightener is activated by a propellant gas which acts in a pressure chamber and has a specific working pressure so that the driving part is moved in the driving direction and the belt reel and the seat belt are driven in a retraction direction. A load limiter allows decelerated extraction of belt webbing with energy consumption after tightening of the seat belt and during blocking of the belt reel, can also be connected to the belt reel. An excessive load due to the applied seat belt on the vehicle occupant's body during this forward displacement is therefore avoided, as the seat belt protracts due to the decelerated extraction of belt webbing.
In the prior art, the activation of the pretensioner can on occasion adversely interfere with the normal operation of the blocking process of the belt reel and the operation of the load limiter of the seat belt. If the retensioning process is initiated early, for example ignition of the propellant gas within 6 ms after the crash, the piston is able to complete the working stroke, and the residual pressure in the guide tube neither influences the blocking process nor operation of the load limiter. During normal initiation (for example normal ignition) within a period of up to about 15 ms after the crash, the driving part, for example the linearly driven piston, can not complete the stroke and hence a residual gas is remaining in the guide tube. The residual pressure results in a force that additionally acts on the force limiter and can cause an unacceptable high load of the belt webbing on the body of the vehicle occupant during his forward displacement. If the working pressure is initiated even later, for example due to ignition of a propellant gas within a period of about 15 ms to 25 ms after the crash, preliminary and/or principal locking of the blocking device can occur prior to tightening but is eliminated again on tightening. If a principal blocking pawl is used, it is removed again from the blocking teeth of the belt reel during tightening. The vehicle occupant's body begins to be displaced forwardly owing to the late initiation of the tightening process and therefore exerts an increased force via the belt webbing and the belt reel on the driving part counter to the retensioning direction. The working stroke of the driving part is limited so that an excessive build-up of pressure is brought about at its rear in the pressure chamber by the propellant gas. This can lead to rupture of the tightener drive system resulting in a rapid drop in pressure and, owing to the forward displacement of the vehicle occupant, the rotation of the belt reel is accelerated such that it is no longer possible to control the preliminary and principal blocking. The webbing is then unwound without deceleration during the forward displacement of the vehicle occupant. The risk of rupture of the tightener drive system also arises if the working pressure is initiated later, i.e. after 25 ms after the crash. A high load which can be as much as 1 kN and higher is applied to the driving part owing to the forward displacement of the vehicle occupant's body. An excessive working pressure is then created in the smallest space, since the driving part hardly moves or only slightly. A rapid reversal of the direction of rotation of the belt reel also occurs here. Therefore, control of the principal locking of the blocking device is no longer possible and the belt webbing is also extracted without deceleration.